One known method for abating certain diesel engine exhaust constituents is by use of an exhaust after-treatment system that utilizes Selective Catalytic Reduction (SCR) of nitrogen oxides. In a typical SCR system, diesel exhaust fluid (DEF), which may include urea or a urea-based water solution, is mixed with exhaust gas before being provided to an appropriate catalyst. In some applications, the DEF is injected directly into an exhaust passage through a specialized injector device. In the case of urea, the injected DEF mixes with exhaust gas and breaks down to provide ammonia (NH3) in the exhaust stream. The ammonia then reacts with nitrogen oxides (NOx) in the exhaust at a catalyst to provide nitrogen gas (N2) and water (H2O).
As can be appreciated, SCR systems require the presence of some form of DEF sufficiently close to the engine system such that the engine can be continuously supplied during operation. Various DEF delivery systems are known and used in engine applications. In known DEF injection systems, a reservoir is installed onto a vehicle for containing the DEF, which is drawn from the reservoir and delivered in metered amounts to the engine exhaust system. The reservoir has a finite urea capacity such that periodic replenishment of the DEF within the reservoir is required.
In certain engine applications where prolonged operation at steady or quasi-steady exhaust flow conditions is present, for example, in engines operating generators, stationary applications, locomotives, hybrid drive vehicles and the like, urea deposits and deposit growth rates in certain regions of urea mixers tend to concentrate in particular, limited areas of the mixer. Such deposits limit the ability to dose urea that is free of deposits at higher rates, essentially limiting the urea delivery capability of the system.